Various arrangements for measuring lightning currents have been suggested in the patent literature.
JP 07043392 discloses a Faraday sensor facilitating measurements of lightning currents by measuring the angle of rotation of linearly polarised light propagating in a magnetic field which is generated by the lightning current.
The Faraday sensor suggested in JP 07043392 comprises a light emitting device from which light is launched into an optical fibre, a rod lens and a rectangular parallelepiped polarizer so as to produce linearly polarized light. The linearly polarized light impinges on a rectangular parallelepiped magneto-optical field sensor which rotates the plane of polarization of the measuring light by an angle β under influence of the magnetic field. The rotational angle β is converted into an intensity of light by means of a rectangular parallelepiped analyzer disposed on the output side of a magneto-optical element. After having left the magneto-optical element the light passes through a rod lens and an optical fibre before reaching and being detected by means of an E/O converter, such as for example a photodiode.
It is a disadvantage of the Faraday sensor suggested in JP 07043392 that external magnetic fields, i.e. magnetic fields other that the magnetic field generated by the lightning current, may easily influence measurements performed by the sensor suggested in JP 07043392. Since the Faraday sensor suggested in JP 07043392 is sensitive to non-lightning related magnetic fields correct determination of the lightning current may be difficult, if not impossible.
It is an object of the present invention to provide a sensor assembly which is insensitive to external and thereby disturbing magnetic fields. By external and disturbing magnetic fields are meant magnetic fields other than the magnetic fields generated by the conductor carrying the current to be measured.
It is further object of the present invention to provide a sensor assembly incorporating a compensation method and an associated compensation arrangement for compensating for aging of the optical components forming the sensor assembly.
It is a still further object of the present invention to provide a signal processing method and an associated signal processing arrangement for processing signals from the sensor assembly.